1. Field of the Invention
The present invention relates to a booster circuit, and more particularly to a booster circuit which boosts a power supply voltage in a semiconductor device.
2. Description of the Related Art
Semiconductor devices such as a nonvolatile semiconductor memory need a high voltage (several volts to 20 volts) as a write voltage for writing data. A nonvolatile semiconductor memory then generates a write voltage to be applied to memory cells by boosting a relatively-low power supply voltage supplied from outside by using a booster circuit such as a charge pump circuit.
A charge pump circuit including a capacitor and a transfer gate has been known. The capacitor charges and discharges an electric charge. The transfer gate repeats an operation of transferring the charge generated by the capacitor to the output line and an operation of stopping the transfer, thereby boosting the voltage on the output line (for example, see Japanese Patent Application Laid-Open No. 2003-249076).
A metal oxide semiconductor (MOS) transistor having a low threshold voltage is employed as the transfer gate used in such a charge pump circuit, in consideration of an increase in the threshold voltage due to a body effect during boosting.
If the threshold voltage of the transfer gate shifts to higher voltages due to manufacturing variations, etc., the transfer period of the charge by the transfer gate may decrease. In such a case, the period until the boosted voltage reaches a desired voltage value increases, and the operation period of the charge pump circuit needs to be increased accordingly. This causes a problem of increased power consumption. If the threshold voltage of the transfer gate shifts to lower voltages, the boosting speed may become too fast. This can cause a problem of ringing of the boosted voltage about the desired voltage value.